The present invention pertains to the field of semiconductor lasers. Semiconductor lasers are generally heterojunction devices fabricated in multiple layers of gallium arsenide, GaAs. This rather complex multiple junction permits laser operation to occur at low current densities, thus dissipating low powers and enabling room temperature laser operation. The laser light is produced in the GaAs layer, which has the narrowest band gap, by the recombination of the electrons and holes produced by the dc current applied as an input. The wavelength of the laser light is determined by the GaAs band gap and lies between 0.89 and 0.90 micrometers. The exact wavelength is further restricted by the requirement that the length of the laser be an integral multiple of the wavelength, .lambda./2, of the laser light produced. In order to produce laser powers of about 200 milliwatts, the laser volume must be large so that the laser will be approximately 1000 micrometers long.
A laser that is 1000 micrometers long extends over 1000 wavelengths. Such a long laser can meet the multiple .lambda./2 length requirement at a large number of closely spaced wavelengths between 0.89 and 0.90 micrometers. Thus, such a laser would simultaneously produce light of many wavelengths, which could be detrimental to a high data rate communications system. Commercial lasers that can tolerate low output power have overcome this multilasing problem by making the laser short, approximately 100 micrometers, so that lasing can occur at only a single wavelength. Complex Bragg reflectors have also been used to achieve the same purpose.
The present invention provides single wavelength lasing while permitting long lasers with high power. A preferred embodiment of the invention consists of an active section, which provides the laser energy, and a passive section, which aids in restructuring the wavelength of operation. Thus, there are two cavities of different lengths. The laser can operate only at wavelengths which meet the requirement that each cavity be an integral number of half wavelengths long. By judicious choice of active and passive cavity lengths, single wavelength laser operation is achieved. The technique should make a significant contribution to the future of semiconductor laser communication systems.
In this regard, semiconductor gallium arsenide (GaAs) lasers have a number of unique characteristics that make them ideally suited for spaceborne communications systems as well as for fiber-optic ground communications. These lasers are small in size, require low voltage and low power for operation, have demonstrated long life, and can readily be modulated for use in high data rate systems. Integrated optical systems are under development which incorporate on a single gallium arsenide substrate the semiconductor laser as well as all other components required in a communications subsystem.
Previous work in the field that is relevant to the present invention includes U.S. Pat. No. 4,054,363 by Y. Suematsu for Multi-Heterostructure Wave Guide Type Optical Integrated Circuitry, Oct. 18, 1977. The patent discloses an optical integrated semiconductor laser incorporating a cavity resonator positioned adjacent to the laser diode. This patent shows structure that is somewhat similar to that presently disclosed as a preferred embodiment, but it does not contemplate the concept of combined active and passive segments of the cavity with a three-mirror configuration.
U.S. Pat. No. 3,948,583 to P. Tien for Isolation of Passive Devices and Integration with Active Devices in Optical Wave Guiding Circuits, Apr. 6, 1976, teaches the general principles of a GaAs laser diode including a cavity having both active and passive segments. The transition between active and passive areas is accomplished by a sloping transistion of the upper substrates rather than by a sharp step.
U.S. Pat. No. 4,212,020 to A. Yariv et al for Solid State Electro-Optical Device on a Semi-Insulating Substrate, July 8, 1980, illustrates a dual heterostructure laser fabricated from GaAs with a fairly sharp step in the upper structure of the integrated circuit. U.S. Pat. No. 4,136,928 to R. Logan et al for Optical Integrated Circuit Including Junction Laser with Oblique Mirror, Jan. 30, 1976, discloses a double heterostructure laser device incorporating a plurality of active regions which are separated and isolated by passive regions through a two step etching process.
U.S. Pat. No. 3,868,589 to S. Waung for Thin Film Devices and Lasers, Feb. 25, 1975, discloses an integrated optical laser in which the properties of optical elements are modified by structural properties in adjacent layers. U.S. Pat. No. 3,978,426 to R. Logan for Heterostructure Device Including Tapered Optical Couplers, Aug. 31, 1976, is of interest primarily with respect to the embodiment illustrated in FIG. 3. An active segment is created to the left of step 19 with a passive segment located to the right. The active structure has a tapered termination instead of an abrupt step. U.S. Pat. No. 4,128,815 to H. Kawaguchi et al for Single Traverse Mode Operation in Double Heterostructure Junction Laser, Dec. 5, 1978, is of general interest with respect to the concept of utilizing an optical wave guide as a controlling means for a single mode operation. U.S. Pat. No. 4,156,206 to L. Comerford et al for Grating Coupled Waveguide Apparatus, discloses a stepped base double heterostructure laser that has two etched steps such that the active layer of the laser aligns with the corrugated waveguide. The waveguide has a grating. U.S. Pat. No. 4,230,997 to R. Hartman et al for Buried Double Heterostructure Laser Device, Dec. 28, 1980, does not use the integrated optics techniques of the invention disclosure. U.S. Pat. No. 4,190,813 to R. Logan et al for Strip Buried Heterostructure Laser, Feb. 26, 1980, has opposing band gap cladding layers. The patent is not directed to the integrated optics techniques. U.S. Pat. No. 4,162,460 to S. Gonda for Optical Circuit Element, July 24, 1979, has a distributed feedback portion that is a Bragg reflector having periodically formed corrugations.
Somewhat relevant to the preferred embodiment of the present invention described herein are writings by L. Coldren et al of Bell Telephone Laboratories, and Y. Suematsu et al of Tokyo Institute of Technology in the Applied Physics Letters of Mar. 1, 1981 and the IEEE Journal of Quantum Electronics of Aug. 8, 1977, respectively. The former manuscript is entitled Monolithic Two-Section GaInAsP/InP Active-Optical Resonator Devices Formed by Reactive Ion Etching, and relates that narrow, high-aspect-ratio grooves formed by reactive ion etching are useful as partially transmissive mirrors for coupled active laser-detector, laser-modulator, and laser-etalon two-section monolithic devices, wherein experimental results emphasized control of the longitudinal mode spectrum by active etalon action. The latter manuscript is entitled Axial-Mode Selectivities for Various Types of Integrated Twin-Guide Lasers, and relates theoretical dependencies of gain to wavelength in double-resonator, distributed Bragg reflector, and two tandem-connection types of integrated twin-guide lasers, for axial-mode selectivity.
Single mode operation is not contemplated by the effort of others in the integrated optics field of double heterostructure structures. The present invention overcomes the limitations of the previous level of technology by providing for single longtitudinal mode operation.